1. Field of the invention
The present invention relates to a transmission shift control apparatus for controlling an automatic transmission of a motor vehicle.
2. Discussion of the Related Art
In the field of a motor vehicle, there is known an automatic transmission which has planetary gear sets and a plurality of frictional coupling devices such as clutches and brakes and in which selected ones of the frictional coupling devices are engaged to connect selected elements of the planetary gear sets to each other or to stationary members, to thereby selectively establish speed or gear positions (operating positions) of the transmission. In such an automatic transmission, predetermined shift patterns are stored in a memory and used to determine whether the transmission should be shifted up or down from the presently established operating position to another. Each shift pattern is determined in relation to the actual running condition of the vehicle, with the drivability and fuel economy of the vehicle being taken into account. For example, the shift pattern consists of a shift boundary line determined as a function of the running condition of the vehicle. For instance, the shift boundary line represents a predetermined relationship between the running speed of the vehicle and a currently detected opening angle of a throttle valve or other parameter indicative of the current load acting on a vehicle engine. In this instance, the automatic transmission is shifted from one operating position to another on the basis of the detected vehicle speed and throttle opening angle and according to the predetermined shift boundary line, such that an appropriate shifting action of the transmission takes place if a point representing the detected running condition (vehicle speed and throttle opening angle) is moved across the shift boundary line.
The shift boundary line for shifting up the transmission (shift-up boundary line) is usually determined such that when the throttle opening angle is relatively close to its largest value (100% opening), a shift-up action of the transmission is initiated immediately before the engine speed has been raised to a predetermined upper limit, so that the drivability of the vehicle is maximum at an opening angle of the throttle valve close to the largest value.
Generally, the automatic transmission has a certain length of shift response time, namely, a time delay from the moment at which the transmission is commanded to be shifted to the moment at which a shifting action of the transmission is actually initiated. During this shift response time period, the engine speed tends to vary due to a change in the temperature or pressure of the intake air (which may be fed from a supercharger), and may rise above the permissible highest level or upper limit during the shift-up action of the transmission. In view of this drawback, there has been proposed to provide the shift control apparatus with a learning shift-point compensating device as disclosed in JP-A-63-30640, which is adapted to effect learning compensation or adjustment of the shift-up boundary line to change the shift-up boundary speed of the vehicle, so that the engine speed which varies during the shift-up action of the transmission is held below a predetermined upper limit. This learning compensation or adjustment is effected if the highest or peak value of the varying engine speed during the shift-up action is outside a predetermined permissible range. The compensated shift-up boundary is stored in a memory and subsequently used to determine whether the same shift-up action of the transmission should take place.
The automatic transmission usually employs a one-way clutch which permits a shifting action to establish a given gear position by engaging or releasing an appropriate hydraulically operated frictional coupling device. However, the automatic transmission may be adapted to effect a so-called "clutch-to-clutch shift" to establish a certain gear position, without using a one-way clutch. The clutch-to-clutch shift is achieved such that one and the other of two hydraulically operated frictional coupling devices are concurrently released and engaged, respectively, to shift the transmission from one operating gear position to another. This arrangement makes it possible to reduce the size and weight of the transmission, owing to the elimination of the one-way clutch. In the clutch-to-clutch shift of the transmission, the releasing pressure of the above-indicated one frictional coupling device or the engaging pressure of the above-indicated other frictional coupling device is regulated so that both of the two frictional coupling devices temporarily have engaging torques during a suitable period of time of the clutch-to-clutch shift.
If the timing of decreasing the engaging torque of one of the two frictional coupling devices and the timing of increasing the engaging torque of the other frictional coupling device are not suitably matched, the motor vehicle will suffer from some problems. For example, the speed of the vehicle engine will have a temporary overshoot if the rate of decrease of the engaging torque of the frictional coupling device to be eventually released is excessively high or if the rate of increase of the frictional coupling device to be eventually engaged is excessively low. Alternatively, the automatic transmission will have a so-called "tie-up", namely, an abrupt temporary drop in its output torque, if the rate of decrease of the engaging torque of the frictional coupling device to be released is excessively low or if the rate of increase of the engaging torque of the other frictional coupling device is excessively high. Thus, the engine speed may suffer from an excessive rise during the shift response time period due to an excessively high rate of decrease of the releasing pressure of the coupling device that should be eventually released, or due to an influence of a control system for controlling the coupling device in question. Therefore, where the transmission shift control apparatus is provided with the learning shift-point compensating device, the operation of this device to change the shift-up boundary line so as to maintain the engine speed below the upper limit tends to be difficult and unstable.
The transmission shift control apparatus may be provided with a learning hydraulic pressure compensating device adapted to effect learning compensation or adjustment of the hydraulic pressure or pressures of the frictional coupling device or devices so as to permit the clutch-to-clutch shift of the transmission to be smoothly achieved, without an excessive amount of engine overshoot or or an excessive amount of tie-up of the frictional coupling devices. The operation of this learning hydraulic pressure control device may cause a change of the engine speed or engine overshoot amount during the clutch-to-clutch shift of the transmission in which the hydraulic pressure is compensated by the learning hydraulic pressure control device. This change of the engine speed or engine overshoot amount may also cause unstable operation of the learning shift-point compensating device.